1. Field of the Invention
This invention relates to a quick-drying plaster. In particular, the invention relates to a method for increasing the drying rate of set plaster to form dry plaster. The invention also relates to a gypsum-containing mixture that, when combined with an appropriate quantity of water, forms set plaster that dries quickly to form dry plaster.
2. Description of Related Art
Gypsum products, such as wallboard, castings, and fireproofing, are formed by depositing an aqueous slurry of calcined gypsum on a substrate, such as paper for formation of wallboard, or in a mold or form. The calcined gypsum in the slurry becomes hydrated and is allowed to set, or harden. The hardened material then is dried, whether at ambient conditions or elevated temperature, until excess water is removed and dry plaster is obtained.
Quick and efficient production of dry plaster is desirable. Therefore, a plaster slurry that dries quickly is preferred. Quick drying saves time during manufacturing and reduces energy costs if heated drying is used. Overdrying, which can occur at the edges and thin parts of a product, typically reduces the strength of the dried product. Overdrying causes calcination of the gypsum dihydrate and reduces the strength of the product. Therefore, a product that dries quickly is less likely to give rise to a portion weakened by overdrying.
Typically, it is preferred that plaster be as light as possible, while maintaining strength and other desirable properties and characteristics. Use of plaster products having reduced density makes those products easier to work with and transport, for example.
One way of reducing the density of dry plaster is to introduce foam into the aqueous gypsum slurry from which the plaster is formed. Examples of such products are disclosed in U.S. Pat. Nos. 3,989,534, 4,518,652, and 6,171,388. Such products require use of film formers, such as guar gum, bentonite, polyvinyl alcohol, or latex polymers.
The rate at which set gypsum can be dried to form dry plaster is commercially important. Thus, it is desirable to achieve a rapid drying rate. Drying set gypsum at elevated temperature is effective in increasing drying rate as compared with drying at ambient temperature. Hence, kilns, ovens, and other devices are used to dry set gypsum to form plaster products. However, overdrying, which causes loss of strength, is more prevalent when heated drying is used. Thus, care must be taken to ensure that no portion of the set gypsum is overdried. The above-described prior art products exhibit standard drying rates and are susceptible to overdrying.
The invention is directed to a gypsum-containing mixture, to quick-drying plaster made therefrom, and to a method for increasing the drying rate of plaster. The plaster dries more quickly by including fatty alcohol sulfate in the slurry from which the plaster is made.
The invention is directed to a method for increasing the drying rate of plaster and to the plaster thus made. Resulting plaster of the invention can be quickly and efficiently dried, yet retains desirable properties and characteristics, such as compressive strength and fire resistance. Plaster of the invention is particularly suited for forming castings and as fireproofing, such as is found in safes and fire-resistant containers. The invention also is directed to a gypsum-containing mixture which, when mixed with an appropriate quantity of water, forms an aqueous slurry that can be quickly dried to form dry plaster.
Skilled practitioners recognize that gypsum, calcium sulfate hemihydrate, which is sometimes called stucco, becomes hydrated in water to form an aqueous slurry of calcium sulfate dihydrate. The slurry hardens, or sets, to form a solid identified herein as set plaster. Any remaining excess water then is removed by drying, with or without heating, to form dry plaster.
The inventors have discovered that addition of fatty alcohol sulfate to gypsum slurry increases the drying rate of the plaster made from the slurry and yields plaster having desirable properties and characteristics. The alcohol sulfate forms foam when the mixture is hydrated during formation of the aqueous slurry of gypsum in water. Slurry containing alcohol sulfate has essentially the same flow characteristics upon hydration as gypsum alone. The resulting foam-containing slurry can be dried quickly to form dry plaster without destroying the hydrated gypsum crystal, thus retaining desired properties and characteristics of dry plaster. In particular, the fire resistance of the plaster is retained.
The inventors also have discovered that quick-drying plaster of the invention can be made lightweight, yet exhibit better compressive strength, and dry more quickly, than known foamed plaster, by adding polyvinyl alcohol to the slurry.
Addition of fatty alcohol sulfate to plaster in accordance with the method of the invention accelerates the drying rate of both foamed and unfoamed plasters. Although the inventors do not wish to be bound by theory, it is believed that addition of fatty alcohol sulfate causes plaster to dry more quickly because the alcohol moiety facilitates evaporation of water.
In accordance with the invention, the drying rate of plaster is increased by including alcohol sulfate in the slurry from which the plaster is made. For example, gypsum and alcohol sulfate can be combined with water to hydrate to gypsum and form an aqueous slurry, which may also contain other compounds. The slurry is hardened. In accordance with the invention, the hardened slurry dries quickly to form dry plaster without adversely affecting the gypsum crystal. The plaster may be made lightweight by including polyvinyl alcohol in the slurry.
Both naturally-occurring and synthetic forms of gypsum are suitable for use in the invention. Gypsum, or calcium sulfate hemihydrate, is available commercially under many names including inter alia molding plaster, statuary plaster, plaster, plaster of paris, and gypsum stucco. Either xcex1-plaster or xcex2-plaster may be used in the method of the invention. Skilled practitioners recognize that xe2x80x9csyntheticxe2x80x9d gypsum is recovered from certain flue gas desulfurization operations and differs from naturally-occurring gypsum because it has a different, typically round, particle shape.
Skilled practitioners recognize that divers materials may be present in dry gypsum or in the aqueous slurry. Molding plaster contains a small quantity of calcium chloride; for example. Additives include colorants; dispersants such as Lomar D (naphthalene sulfonate, available from GEG Specialty Chemical) (for control of fluidity); vermiculite (for fire resistance); defoamers such as Foamaster PD#1, available from GEG Specialty Chemical (to control foam in the slurry); foaming agents such as polyvinyl alcohol (to foam the slurry); retarders, such as sodium citrate; and the like. Such materials may be added to the aqueous slurry or to the mixture of the invention in quantities that do not adversely affect the basic nature of the invention. Skilled practitioners recognize that some types, such as molding plaster and xcex1-plaster, will dry more quickly than other types, because the initial water demand is lower.
Alcohol sulfate suitable for use in the mixture of the invention has an alkyl moiety having between about 8 and about 20 carbon atoms. Preferably, the alcohol sulfate has an alkyl moiety of between about 10 and about 16 carbon atoms. Such alcohol sulfate is commercially available. One product line of suitable alcohol sulfates is available from Witco under the trade names Supralate(copyright) and Dupanol(copyright). These products are fatty alcohol sulfates, typically combined with sodium as the balancing ionic moiety.
Suitable alcohol sulfates (and the trade name under which they are available from Witco) include sodium lauryl sulfate (Supralate(copyright) M E dry, a dry product), also known as sodium dodecyl sulfate; sodium tetradecyl sulfate (Dupanol(copyright) WA Q E); sodium octyl/decyl sulfate (Supralate(copyright) W N); sodium octyl sulfate (Supralate(copyright) S P); and diethanolamine lauryl sulfate (Supralate(copyright) E P). Sodium lauryl sulfate formerly was available from duPont under the tradename Dupanol(copyright). The products comprising sodium as the balancing ion are preferred; sodium lauryl sulfate, particularly Supralate(copyright) M E, is more preferred.
Skilled practitioners recognize that the fatty alcohol sulfates described herein are solids. Thus, it is anticipated that the mixture of the invention will be a dry mixture, and alcohol sulfate will be introduced into the slurry in this way. Dry mixtures are particularly advantageous, as they can be easily transported and stored. However, fatty alcohol sulfates are available in liquid form, whether alone or as, for example, an aqueous solution or suspension. Such liquid products are not suitably incorporated into dry mixtures. However, skilled practitioners recognize that the mixture of the invention can be formed with such liquid phase alcohol sulfate by combining gypsum with the alcohol sulfate and water to form a slurry. Introduction of the alcohol sulfate directly into the slurry is within the scope of the claimed invention.
The quantity of alcohol sulfate added is between about 0.005 and about 0.2 weight %, based on the weight of the gypsum; and preferably between about 0.008 and about 0.15 weight %; and more preferably between about 0.01 and 0.10 weight %. Lesser quantities can be used, but are not likely to provide the quick drying and crystal protection desired. Greater quantities may introduce undesirable foaming and not yield a cost effective marginal drying rate increase. With the guidance provided herein, skilled practitioners will be able to select an appropriate quantity of alcohol sulfate.
Skilled practitioners recognize that the fatty alcohol sulfates described herein are solids. Thus, it is anticipated that the mixture of the invention will be a dry mixture, and alcohol sulfate will be introduced into the slurry in this way. Dry mixtures are particularly advantageous, as they can be easily transported and stored. However, fatty alcohol sulfates are available in liquid form, whether alone or as, for example, an aqueous solution or suspension. Such liquid products are not suitably incorporated to dry mixtures. However, skilled practitioners recognize that the mixture of the invention can be formed with such liquid phase alcohol sulfate by combining gypsum with the alcohol sulfate and water to form a slurry. Introduction of the alcohol sulfate directly into the slurry is within the scope of the claimed invention.
Slurry made by combining mixture of the invention with water in quantity sufficient to hydrate the gypsum and provide a workable slurry has flow characteristics, such as pouring consistency, comparable to those of gypsum slurry alone. Skilled practitioners recognize that there exist a number of tests for determining the flow characteristics of gypsum slurry. For example, one can use a consistometer, a device known to practitioners. One also can determine pouring consistency by a free pour of gypsum slurry onto a smooth inert surface such as glass from a height of, e.g., about 1-4 inches, preferably about 2 inches. It is convenient to determine the weight of water necessary per weight of the gypsum to yield a pat of predetermined diameter.
Any suitable method can be used to determine pouring consistency. For example, one can determine the pat diameter obtained when gypsum is hydrated with a given quantity of water. Comparison of such diameters for divers gypsum sources will provide a relative measure of fluidity for the sources.
Dry plaster of the invention is made by drying set gypsum to remove excess water. The set product can be dried under ambient conditions or by heating in a kiln, oven, or other suitable device known to skilled practitioners. Typically, drying temperatures between about 125 and about 225xc2x0 F. are utilized for a time sufficient to achieve the desired degree of dryness at the drying temperature selected. For example, drying set plaster to a pre-selected moisture level may take about 225 minutes at 150xc2x0 F., but only 60 minutes at 200xc2x0 F. With the guidance provided herein, a skilled practitioner will be able to select suitable drying conditions.
Plaster of the invention can be dried in any manner known to skilled practitioners. Plaster of the invention will dry more quickly at a given temperature than known plasters without suffering adverse effects of overdrying at the edges and thin portions. As skilled practitioners recognize, overdrying causes disruption of the dihydrate gypsum crystal and dehydration thereof to calcium sulfate hemihydrate, which has a very low compressive strength.
The inventors also have discovered that including polyvinyl alcohol in the slurry can form particularly lightweight plaster. The polyvinyl alcohol foams the plaster sufficiently to reduce the density thereof However, in combination with the fatty alcohol sulfate, the resultant product dries quickly and has particularly high compressive strength. In accordance with the invention, the compressive strength is about 25% higher than plaster foamed to the same density by known methods, preferably about 40% higher, and more preferably about 50% higher.
Polyvinyl alcohol having any degree of hydrolysis, typically at least about 75%, is suitably used in the invention. Polyvinyl alcohol is available as aqueous solutions in varying concentrations and as solid. As set forth above, solid polyvinyl alcohol can be added to a dry mix to form a dry mixture, and liquid polyvinyl alcohol solution can be introduced as part of formation of the slurry.
Skilled practitioners recognize that polyvinyl alcohol having a degree of hydrolysis less than about 88% is freely soluble in water at about 150xc2x0 F. However, the solubility of polyvinyl alcohol decreases with increasing degree of hydrolysis, so higher temperatures are required for complete dissolution. Thus, a degree of hydrolysis of less than about 88% is preferred.
The following examples illustrate the invention.